1. Field of the Invention
The present invention relates to a fluid dynamic bearing, a method of manufacturing the fluid dynamic bearing, a rotating device, and a disk drive device, and in particular, to a technique for stabilizing the rotational accuracy of a rotating body.
2. Description of the Related Art
Recently, in disk drive devices such as HDDs (Hard Disk Drives), the rotational accuracy of a rotating body has been drastically improved by being supported with a fluid dynamic bearing provided in the disk drive device. The fluid dynamic bearing is provided with dynamic pressure grooves for generating dynamic pressure in part of the rotating body thereof, such as the shaft, so that dynamic pressure is generated by the interaction with a lubricant such as oil, which is filled around the rotating body, during the rotation of the rotating body. Through the generated dynamic pressure, the fluid dynamic bearing makes the rotating body substantially float in the lubricant during the rotation of the rotating body so that the rotating body is supported in a non-contact manner with ambient components. With such a background in which the bearing accuracy of the fluid dynamic bearing has been improved, the disk drive device has been required to have a higher density and a greater increased capacity for data to be stored. For example, in a disk drive device capable of reading/writing data magnetically, a recording disk on which recording tracks are formed is rotated at a high speed. The magnetic head executes the reading/writing data while tracing the recording tracks of the recording disk with a slight gap between both. Because the size of the recording disk used in such a disk drive device is limited, the width of the recording track has been required to be small in order to realize a substantial higher density and a greater increased capacity. In accordance with the width of the recording track, the gap between the magnetic head and the recording disk has also been required to be small.
With the realization of a higher density and a greater increased capacity, the disk drive devices have been mounted in various apparatuses. With such a trend, weight saving of the disk drive device has become an important issue. To deal with such a demand for the weight saving, it has been investigated whether components of a fluid dynamic bearing may be made of resins. For example, Japanese Patent Application Publication No. 2001-107946 discloses a fluid dynamic bearing whose flange is made of a resin, the flange having thrust dynamic pressure grooves for generating a dynamic pressure in the thrust direction, and a method of manufacturing the fluid dynamic bearing in which the flange is fixed to the shaft when the flange is molded.
However, when a resin flange with thrust dynamic pressure grooves is molded integrally with a shaft by the manufacturing method disclosed in Japanese Patent Application Publication No. 2001-107946, the perpendicularity between the flange and the shaft is not stabilized in many cases. That is, when one intends to mold a flange by filling the surrounding area of a shaft, the shaft being composed of a material different from a resin, with a high-temperature molten resin, the temperature of the shaft is increased from room temperature to, for example, one hundred and tens degrees Celsius, thereby causing the shaft to thermally expand. In this state, the flange is to be formed in accordance with the shape of the mold and fixed to the shaft. Thereafter, the boundary surface between the shaft and the resin contracts non-uniformly in the course where the temperature of the shaft returns to room temperature, thereby the flange is sometimes tilted. Because the contraction in this case varies in every molded article, the perpendicularity between the shaft and the flange also varies in every molded article. Such variation in the perpendicularity between the shaft and the flange causes surface fluctuation of the flange when rotated on the basis of the shaft. The surface fluctuation of the flange means the slope of the surface on which the thrust dynamic pressure groove has been formed. If a shaft with a flange, the surface of which is greatly tilted, is installed in a fluid dynamic bearing, the space between the tilted surface of the flange and the surface facing the tilted surface in the axial direction becomes disproportionate in accordance with the slope of the flange. The disproportion of the space also varies greatly in accordance with rotational movement. Because thrust dynamic pressure generated by rotation of the flange is decreased in the area where the space between the flange on which the thrust dynamic pressure grooves has been formed and the surface, facing the flange in the axial direction, is large whereas increased in the area where the space is small, the thrust dynamic pressure becomes disproportionate. Such disproportion of the thrust dynamic pressure may cause a disproportionate load in the radial bearing unit and may deteriorate the rotational accuracy of the shaft. In the disk drive device mounted with such a fluid dynamic bearing, the rotational accuracy of which is disproportionate, the disproportion of the rotational accuracy may deteriorate the error rate in reading/writing data when the width of the recording track is small. That is, there is a problem that the performance of the surface fluctuation of the flange eventually becomes an obstacle to a higher density and greater increased capacity disk drive device.